The removal or reduction of noxious constituents of coal combustion products, or flue gas, has been attempted since the beginnings of the Industrial Revolution in eighteenth century England. Without air pollution controls, the combustion of coal produces significant quantities of carbon monoxide (CO), carbon dioxide (CO.sub.2), oxides of nitrogen (NO.sub.x), oxides of sulfur (SO.sub.x), volatile organic compounds (VOC's), other hazardous air pollutants (HAP's), and fly ash, or particulate materials (PM). PM was the first targeted pollutant in flue gas, followed by SO.sub.x and NO.sub.x. Currently, there is a global effort to limit emissions of CO.sub.2, due to its role as a greenhouse gas.
Current air pollution (A/P) control technologies are primarily "segmental" in that there is a separate and distinct approach for limiting the emission of each deleterious constituent of the exhaust stream. The subsequent disposal of the resulting waste products for each pollutant creates separate waste streams. While the reduction of PM and SO.sub.x are typically achieved in the post combustion phase, and NO.sub.x emissions are typically treated during the combustion phase, some novel designs attempt to integrate SO.sub.x and NO.sub.x control within the combustion zone.
Conventional PM removal from post combustion flue gas emissions centers around the use of cyclonics; a method of removing ash via centrifugation and gravity. The flue gas passes through an array of cyclones called a multiclone. A collection system entraps and contains the ash, which must be periodically collected and disposed of as a hazardous waste.
There is little practical application for coal ash that is collected. Since it is considered a hazardous waste, so it cannot simply be dumped in a sanitary landfill. Furthermore, the ash content of coal differs from batch to batch, and is further complexed by a myriad of combustion variables, so it is difficult to optimize and simplify the operation of the multi clones. Multi clones operate by creating a zone of low pressure and draw electrical power for that purpose. It is difficult, if not impossible, to adjust the level of the pressure differential required to remove the ash under constantly changing conditions.
The multiclone is capable of efficiently removing PM above a specific density and size. However, finer, lighter PM remain in the flue gas stream. In order to prevent the lighter PM from being emitted from the stack, coal combusting facilities often employ electrostatic precipitators. Electrostatic precipitators maintain an electrical charge opposite to the ionization of the PM in the exhaust stream to draw the PM to an electrically charged grid. When the grid becomes coated with solids, it is cleaned, usually with a system employing hot, pressurized water or steam.
Cleaning electrostatic precipitators creates an ash slurry that must be treated and disposed. This process consumes water which must be heated and pressurized. The degree of ionization in the fine PM is in part determined by sulfur content in the coal, so the effectiveness of the grid in attracting PM may vary. Even when electrostatic precipitators work well to remove fly ash from the flue gas, they are not able to remove much of the "fine PM", typically defined as those particles with an aerodynamic diameter of less than 10 microns. To correct this shortfall wet scrubbing is added.
Another method of removing PM from flue gas resulting from solid fuel combustion is the baghouse. A baghouse is a chamber with a fabric filter through which flue gas is passed. The filter entrapped PM is then vacuumed out and disposed as hazardous waste.
Flue gas desulfurization (FGD) is the category name applied to the spectrum of technologies designed to chemically bond the flue gas SO.sub.x, which is primarily sulfur dioxide (SO.sub.2) in flue gas, to calcium, magnesium or other binders. FGD methods differ in the way in which calcium or other binders are delivered to the exhaust stream, and the subsequent treatment or disposition of the spent sorbent (calcium-containing compound). FGD technologies are categorized as wet, dry, or regenerable.
Wet FGD is a post combustion technology that uses either limestone or lime suspended in water as a medium through which the stack gases must pass. Flue gas is sprayed with the calcium-containing slurry which reacts with the SO.sub.x to form a wet, toxic sludge. The sludge formed by the activity of the wet scrubber must then be disposed.
Dry FGD utilizes a lime slurry or soda ash solution injected into a spray dryer. When the solution contacts the SO.sub.x in the flue gas the reactants form a dry waste, some of which can be converted into drywall, or gypsum board.
Regenerable FGD technologies such as the Wellman-Lord process employ a variety of chemical reactions to reclaim the SO.sub.x from the sorbent and convert the SO.sub.x into sulfuric acid, elemental sulfur, or other useful compounds. When purged of its sulfur content, the sorbent can then be reused.
All the variants of sorbent-based FGD are seriously flawed both conceptually and operationally. Sulfur oxides are a very small component of the stack gas mixture, typically comprising less than 0.05% of the mass flow, when excess combustion air is included. The flue gases are in a high entropy state and in motion, making it difficult to achieve the proper stoichiometry for the reaction to take place.
Sorbent-based FGD is, by its nature, a very inefficient way to remove SO.sub.2 from flue gas. The preparation, mixing, and pumping of sorbents requires considerable horsepower. SO.sub.2 removal rates are proportional to the amount of sorbent used and the energy requirements of circulating the sorbent. The flue gases, after being scrubbed, must be reheated for proper drafting. The parasitic energy requirements of a complete FGD and PM removal system can be approximately 4% to 7% of the total plant power output of an electrical generating facility.
The sludge or solid waste resulting from FGD are considered toxic and must be disposed of in a hazardous waste facility. Only a small percentage of the waste is converted into any useful product, which is primarily gypsum board products. Additionally, the gypsum products are low profit items. As a result, the energy costs of making sulfuric acid or elemental sulfur in the regenerable FGD process easily erases the slim profit margins generated in the sales of the gypsum by-products.
Furthermore, wet FGD and Wet Electrostatic Precipitators (WEPs) consume significant quantities of water. This is a highly undesirable feature, because the water must be further processed for recycle or discharge, at a premium cost. While most FGD technologies are post combustion processes, NO.sub.x reduction is achieved primarily through other modifications to the combustion environment.
Staged combustion operates by an initial reduction of the amount of air necessary to achieve complete combustion of the coal. The partial combustion releases nitrogen from the coal and is followed by a second stage that completes the combustion of the fuel. A widely utilized variant of staged combustion called Flue Gas Recirculation simply recycles and reburns a portion of the flue gases to the primary stage of the combustion process to lower the peak flame temperature and reduce the available oxygen, which favors a reduction in NO.sub.x emissions.
Low NO.sub.x burners are commonly designed to facilitate staged combustion in a way that minimizes the inefficiencies inherent in any method that interferes with the stoichiometry of the oxidation process. Low NO.sub.x burners employ a fuel rich primary zone and secondary burnout zone for combustion. Both air flow and fuel flow are split prior to entering the burner, effectively creating the two zones. Nitrogen is dissociated from the coal in the primary zone and does not readily oxidize. The fuel from the primary zone is then more completely oxidized in the secondary zone.
The primary problem with the conventional NO.sub.x reduction technology is that it interferes with the stoichiometry of combustion in an attempt to modify emissions at the expense of efficiency. Consequently, to achieve cleaner emissions some of the heat content of the fuel is lost due to incomplete oxidation and a resultant loss of fuel burning efficiency.
There are several novel technologies that attempt to reduce both NO.sub.x and SO.sub.x in the combustion zone. The Limestone Injection Multistage Burner (LIMB) combines staged combustion with limestone injection. The SO.sub.2 combines with the limestone to create calcium sulfate. Removal rates of SO.sub.2 are lower than with wet FGD, but the LIMB is a less expensive retrofit.
An offspring of LIMB technology is Atmospheric Fluidized Bed Combustion (AFDC). In this fluidized bed process, powdered coal and limestone are injected onto a bed of sand that is fluidized by streams of injected air. Combustion temperature is lowered to achieve NO.sub.x reduction, but higher SO.sub.2 removal is achieved. Both LIMB and AFDC achieve SO.sub.2 and NO.sub.x removal in the combustion zone at the cost of extensively impairing the stoichiometry of the fire and reducing efficiency.
Another relatively new control strategy is called Integrated Gasification Combined-Cycle Technology (IGCG). IGCG employs coal gasification to substantially reduce SO.sub.2 emissions and powers a steam turbine with the flue gas to enhance efficiency. IGCG, however, is itself a thermochemical process depending on coal combustion and so is subject to the all the problems of direct coal burning.
In summation, there are a number of adverse consequences to the spectrum of technologies designed to reduce the levels of pollutants from the exhaust stream of coal combustion, all of which have significant and inherent problems, adding considerable capital, operating or disposal and waste treatment costs that make them undesirable.
PM removal and FGD produce large volumes of hazardous waste in separate streams which cannot be beneficially combined, especially when considered in proportion to fuel consumption and the ash and sulfur content of fuel. These post combustion emission mitigation technologies have considerable parasitic energy requirements that consume excess fuel and thereby raise greenhouse gas emissions per unit of available power. Mitigation efforts centering around the combustion zone impair the stoichiometry of the reaction, lowering boiler efficiency and raising levels of greenhouse gas emissions per unit of available power. Achieving higher removal rates of pollutants incurs higher energy penalties in the post combustion technologies and lowered stoichiometry in the combustion-based technologies. PM below a certain specific density and size cannot be removed with the current methods. All the mitigation technologies are expensive to implement, maintain, and operate; retrofitting installations adds to the expense. There is no direct or ancillary enhancement to the operation of coal combustors deriving from implementation of the current mitigation technologies.